Field of the Invention
1. This invention relates to unsaturated polyester resin compositions used for coating metal substrates and to a method for the manufacture thereof. More particularly, the invention relates to unsaturated polyester resin compositions excellent in fast adhesiveness to metals and in air drying property and therefore, suitable for coating metal substrates such as steel plates and tin plates, and to a method for the manufacture thereof.
2. Description of the Prior Art
Generally, the unsaturated polyester resin is prepared by causing an unsaturated polyester, the product of the dehydration condensation reaction between an acid moiety comprising an .alpha.,.beta.-unsaturated dibasic acid such as fumaric acid, maleic anhydride or itaconic acid and a saturated dibasic acid such as phthalic acid, phthalic anhydride, isophthalic acid or adipic acid and an alcohol moiety such as ethylene glycol, diethylene glycol, propylene glycol or dipropylene glycol, to be dissolved in a polymerizable vinyl monomer such as styrene, vinyl toluene or diallyl phthalate which serves as the crosslinking agent in the reaction. It is polymerized and cured by using, as the polymerization catalyst, an organic peroxide such as benzoyl peroxide or methylethylketone peroxide and optionally using, as the polymerization accelerator, an organic metal salt such as cobalt octenate or cobalt naphthenate. In the unsaturated polyester resin thus produced, while the inner portion of the resin is cured rapidly, the surface portion of the resin exposed to the ambient air long retains its viscousness owing to the polymerization inhibiting activity exhibited by the oxygen in the air and, therefore, is devoid of air drying property and of intimate adhesiveness to metals.
For the purpose of overcoming such various drawbacks suffered by the conventional unsaturated polyester resins, there has been proposed an air-drying unsaturated polyester resin composition excelling in adhesiveness to metals, which is obtained by interesterifying at least one drying oil, semi-drying oil or nondrying oil with a polyhydric alcohol, causing the product of this interesterification to react with a polyhydric alcohol, an unsaturated polybasic acid or a mixture of an unsaturated polybasic acid with a saturated polybasic acid and dicyclopentadiene to synthesize a dicyclopentadiene-type oil-modified unsaturated polyester having an oil length of not more than 30% and dissolving the modified unsaturated polyester in a vinyl group containing compound (Japanese Patent Publication No. 15547/1961). Although this dicyclopentadiene-type oil-modified unsaturated polyester resin composition excels in fast adhesiveness to metals, it has a disadvantage in that it lacks ample air-drying property and, when applied to metal substrates, tends to collect dirt and betray poor workability.
On the other hand, as unsaturated polyester resin compositions excel in air-drying property, there have been proposed unsaturated polyester resin compositions modified with allyl ethers such as trimethylol propane diallyl ether, glycerine diallyl ether and pentaerythritol triallyl ether. Despite their excellent air-drying property, these allyl ether-modified unsaturated polyester resin compositions are still devoid of fast adhesiveness to metals. For this reason, they have been hardly accepted for the purpose of coating metal substrates.
The unsaturated polyester resins produces good results in the coating of metal substrates because of their salient merit that they can be applied in greater thicknesses to substrates than those coating compositions obtained by admixing alkyd resins, acrylic resins, etc., to diluting solvents. For this reason, debut of an unsaturated polyester resin composition which dries fast in air and excels in fast adhesiveness to metals is longed for.
An object of this invention, therefore, is to provide a novel unsaturated polyester resin composition for coating metal substrates and a method for the manufacture thereof.
Another object of this invention is to provide an air-drying unsaturated polyester resin composition which is excellent in fast adhesiveness to metals and in air drying property and, therefore, is useful for the coating of metal substrates and to a method for the manufacture thereof.